A Middle-Upper Miocene fluvial-lacustrine rift sequence in the Song Ba Rift, Vietnam:: an analogue to oil-prone, small-scale continental rift basins

The small Neogene Krong Pa graben is situated within the continental Song Ba Rift, which is bounded by strike-shp faults that were reactivated as extensional faults in Nfiddle Miocene time. The 500 m thick graben-fin shows an overall depositional development reflecting the structural evolution, which is very similar to much larger and longer-lived graben. The basal graben-fill consists of thin fluvial sandstones interbedded with well-oxygenated lacustrine siltstones in the basin centre, while very coarse-grained fluvial sandstones and conglomerates dominate at the basin margins. With increased subsidence rate and possibly a higher influx of water from the axial river systems the general water level in the graben rose and deep lakes formed. High organic preservation in the lakes prompted the formation of two excellent oil-prone lacustrine source-rock units. In the late phase of the graben development sedimentation rate outpaced the formation of accommodation space and fluvial activity increased again. During periods when the general sedimentation rate was in balance with the creation of accommodation space the environment changed frequently between lake deposition and intermittent vigorous fluvial activity. It is likely that the resulting interbedding of fluvial sandstones and lacustrine sediments reflects variations in precipitation. In periods of little precipitation the lakes diminished and lake bottoms became exposed. After heavy precipitation, transverse river systems transported sands from the rift shoulders across the exposed lake bottom and fluvial sands were deposited on lake bottom sediments. Subsequently, lake level rose due to increased water supply from the axial river and the sands were drowned and topped by transgressive lacustrine mudstones. These sandstones mav function as carrier beds, whereas the braided fluvial sandstones and conglomerates along the graben margins may form reservoirs. The K rong Pa graben thus contains oil-prone lacustrine source rocks, effective conduits for generated hydrocarbons and reservoir sandstones side-sealed by the graben faults toward the footwall granites. In addition to the structural and climatic signals recorded by the graben-fitl, sediment partitioning among the partly isolated basins along the rift axis seems to have been important.